This invention relates to a metrological apparatus and to a metrogical method using polarisation modulation of a beam of radiation.
In particular a first and third aspect of the invention relates to a metrological apparatus comprising:
means for projecting a beam of polarised radiation such that the beam can be returned back along substantially the same path;
means for modulating the polarisation of the projected and returned portions of the beam; and
means for detecting the returned portion of the beam after modulation.
Furthermore a second and a fourth aspect of the invention relate to a corresponding metrological method comprising steps of:
projecting a beam of polarised radiation;
returning the polarised beam along substantially the same path;
modulating the polarisation of the projected and returned portions of the beam; and detecting the returned portion of the beam after modulating.
Such an apparatus and method are known from United Kingdom patent specification No. GB 1172668. In the arrangement described in that specification the beam is polarised by a first polarising filter and is then modulated by at least one electro-optical crystal. The returned portion of the beam is modulated by at least one further such crystal having its crystal structure having its x and y axes oriented perpendicularly to those of the first crystal and is then filtered by a second polarising filter having its polarising direction crossed with that of the first filter.
It will be appreciated that, with matched crystals correctly oriented, with the polarising filters correctly oriented and with no stray polarising effects or natural birefringence of the crystals, no radiation will pass through the second polarising filter when the distance along the optical path from the first crystal to the second crystal is an integral number of modulation wavelengths. However, setting up the arrangement to perform perfectly is difficult if not impossible.
A fifth aspect of the present invention is concerned with a metrological apparatus comprising:
means for projecting a beam of radiation such that the beam can be returned to the apparatus;
means for varying the length of the path travelled by the beam;
means for detecting a characteristic of the returned portion of the beam which varies with the length of the path travelled by the beam; and
means for determining when the rate of change of the detected characteristic with respect to the length of the path is substantially zero.
Furthermore, a sixth aspect of the present invention relates to a corresponding metrological method.
Such an apparatus and method are also known from United Kingdom Patent specification No. GB 1172668. In the arrangement described in that specification, the intensity of light passing through a polarising filter is detected, and steps are taken to determine when the detected intensity is a null.
A seventh aspect of the invention is concerned with a metrological method comprising the steps of:
projecting a beam of radiation from a first portion of a structure to a second portion of the structure;
modulating the beam at a modulation wavelength defined with respect to a reference wavelength;
detecting the beam after it has reached the second portion of the structure; and
determining the distance between the first and second portions using the result of the detection step;